The present invention relates to a horn speaker suitable for use in indoor or outdoor broadcastings such as a broadcasting in a hall, a station yard, a playground or the like, the capable of forming a uniform sound field for a number of listeners to permit the listeners to listen to the sound at the same tone quality and clearness and at a high fidelity of reproduction, regardless of the positions occupied by the listeners. More particularly, the invention is concerned with a horn speaker which is improved to suppress the disturbance of radiation impedance and to flatten the frequency characteristics.
Conventional horns incorporate various types of horns such as a radial horn, a conical horn and so forth. The radial horn is designed to generate arcuate wave surfaces in a horizontal plane so that arcuate wave surfaces are propagated in a concentric manner along the inner surface of the horn. This type of horn, therefore, transmits sound in the form of concentric wave surfaces to exhibit a superior directivity in the horizontal direction. However, the directivity in the vertical direction is not so good in this type of horn.
On the other hand, the conical horn disadvantageously suffers a problem of disturbance in the radial impedance characteristics, although it exhibits high directivities in both of horizontal and vertical directions.
FIGS. 1a and 1b show a conical horn which is disclosed in Japanese Patent Laid-open No. 12724/1979. This conical horn is formed of two conical horns in combination and, has lateral wall curves made straight. This conical horn, however, exhibits a large disturbance of radiation impedance.
FIG. 2a shows the shape of side wall of an exponential conical Bessel horn which is a typical conventional horn, while FIG. 2b shows the radial impedance characteristics of this horn. The shape of this horn is given by the following Webster's general equation concerning the Bessel horn. EQU S.sub.M =S.sub.o (1+.alpha.x).sup.n
where,
S.sub.M : cross-sectional area of the horn PA1 S.sub.o : cross-sectional area of throat PA1 .alpha.: divergence coefficient PA1 x: distance from throat
In the above-mentioned equation, the case where n equals to 1 corresponds to the conical horn, the case where n is infinitive (.infin.) corresponds to the exponential horn and the case where n takes a value intermediate between 1 and infinitive corresponds to the Bessel horn.
As will be understood from FIG. 2b, the disturbance of the radiation impedance becomes greater as the value of n gets smaller and, hence, the conical horn exhibits the greatest disturbance of the radiation impedance.